This invention relates generally to joining dissimilar metals by use of a laser and, more specifically, to laser joining gold onto wires of an endoluminal stent.
A stent is an elongated device used to support an intraluminal wall. In the case of a stenosis, a stent provides an unobstructed conduit for blood in the area of the stenosis. Such a stent may also have a prosthetic graft layer of fabric or covering lining the inside or outside thereof, such a covered stent being commonly referred to in the art as an intraluminal prosthesis, an endoluminal or endovascular graft (EVG), or a stent-graft.
A prosthesis may be used, for example, to treat a vascular aneurysm by removing the pressure on a weakened part of an artery so as to reduce the risk of rupture. Typically, a prosthesis is implanted in a blood vessel at the site of a stenosis or aneurysm endoluminally, i.e. by so-called xe2x80x9cminimally invasive techniquesxe2x80x9d in which the prosthesis, restrained in a radially compressed configuration by a sheath or catheter, is delivered by a deployment system or xe2x80x9cintroducerxe2x80x9d to the site where it is required. The introducer may enter the body through the patient""s skin, or by a xe2x80x9ccut downxe2x80x9d technique in which the entry blood vessel is exposed by minor surgical means. When the introducer has been threaded into the body lumen to the prosthesis deployment location, the introducer is manipulated to cause the prosthesis to be ejected from the surrounding sheath or catheter in which it is restrained (or alternatively the surrounding sheath or catheter is retracted from the prosthesis), whereupon the prosthesis expands-to a predetermined diameter at the deployment location, and the introducer is withdrawn. Stent expansion may be effected by spring elasticity, balloon expansion, or by the self-expansion of a thermally or stress-induced return of a memory material to a pre-conditioned expanded configuration.
Various types of stent architectures are known in the art, including many designs comprising a filament or number of filaments, such as a wire or wires, wound or braided into a particular configuration. Other non-wire stent architectures may comprise a tubular sleeve of metal that is laser cut into an expandable and collapsible design. Often, during implantation of wire stents into a body lumen, the attending surgeon or other member of the surgical team needs to view the positioning of the stent within the lumen using fluoroscopy. Thus, radiopaque markers are often used, a radiopaque marker being any portion of a stent that has a different fluoroscopic reflectance than surrounding portions of the stent. As most stent wires comprise nitinol (a nickel-titanium alloy) or stainless steel, one metal that can serve as such a fluoroscopic marker is gold.
It is known in the art to attach gold to the other metal by electro-plating, by physical attachment such as winding a marker band around a portion of the wire or sliding a gold hypotube over the wire like a sleeve, by ion implantation methods, or by welding methods such as arc-welding. It is also known to weld gold with an electron beam. Electro-plating generally provides only a thin coat of gold having. an undesirably rough surface, and generally results in poor adhesion of the gold to nitinol. Ion implantation methods generally may also have poor adhesion to some metals such as nitinol. Arc-welding methods, although applicable for joining gold to other metals on a large scale, are inapplicable for the small scale of stent construction. Physical attachment is labor intensive and generally is inapplicable to non-wire stents such as laser-cut tubular stents. Physical attachment to wire stents typically comprises threading a marker band hypotube onto the stent wire before or during winding the stent on a mandrel. Electron beam welding equipment is capital intensive and is less flexible that laser beam welding, as electron beam welding requires performing the welding process in a vacuum.
Laser welding of gold to stent wires using a commercially available Nd:YAG (neodymium:yttrium-aluminum-garnet) laser at a standard wavelength of 1.064 xcexcm has been tried with variable success. One problem with performing such laser welding processes is the difficulty in selecting an optimum laser intensity, because the temperature rise in gold is typically much lower than the temperature rise in the metal to which the gold is being welded. Thus, a laser intensity that heats the gold to the proper temperature may heat the surrounding stent wire metal to a temperature that is so high that it causes excessive melting, vaporization, and/or cutting of the small-diameter stent wire to which the gold is to be welded. Similarly, if the wire is inadequately covered by the gold, leaving portions of the wire exposed during the welding step, or if the gold to be welded has a variable thickness, even if the precise intensity for welding one section is selected, that intensity may cause the same melting, vaporization, and/or cutting problems where the wire is exposed or the gold is thinner.
Thus, there is a need in the art to provide a reliable process for joining gold to other metals, specifically for laser processing of gold onto metal stent wires.
In view of the shortcomings of the prior art, the present invention provides a process for joining a first metal, having a first reflectance curve as a function of wavelength, with a second metal, having a second reflectance curve, through the use of a laser. The process comprises placing the first metal in contact with the second metal and exposing at least one of the first metal and second metal to a laser beam having a wavelength selected from an optimal range of wavelengths over which the first reflectance curve and the second reflectance curve essentially overlap. The first metal may be nickel, titanium, iron, or an alloy thereof, and the second metal may be gold or copper. With the wavelengths of the laser beam selected to be over an optimum range, any adverse effect caused by one of the metals heating significantly due to a difference in reflectance is avoided. The laser joining can involve bonding the two metals in some way, such as by welding, cladding, or alloying.
The invention also includes a stent adapted for endoluminal deployment within a body lumen made by the process described above. Such a stent includes one or more first metal elements each comprising a first metal and a radiopaque marker metal attached to one or more portions of at least one element. The radiopaque marker comprises a second metal attached to the first metal by one of: a weld, a clad layer of the second metal over the first metal, an alloy layer of the first metal and the second metal over the first metal, or a combination thereof. The attachment process uses a laser having a wavelength within the optimal range, as described above.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.